The present invention relates to a receiver for identifying a pulse signal in a composite signal transmission system including a multivalue pulse code modulated signal. In more detail, the present invention relates to a receiver for receiving a composite signal including a video signal and a pulse code modulated audio signal arranged in a predetermined sequence and also including a plurality of pulse coded synchronizing signals required for the reproduction of said video signal and audio signal inserted in a predetermined position of said video and audio signal, and for accurately identifying the pulse signals included in the composite signals.
A suitable embodiment of such a composite signal is a still picture broadcasting signal. The receiver according to the present invention is therefore suitable but not limited to use as a receiver for receiving the still picture broadcasting signal.
The still picture broadcasting signal has been disclosed in detail in application Ser. No. 361,581 filed in 1973, now U.S. Pat. No. 3,854,010, and will be briefly explained hereinafter to an extent to help the understanding of the present invention.
In one embodiment of the still picture broadcasting system, the video signal and the audio signal are transmitted alternately acccording to a predetermined sequence. In one mode of signal transmission in said broadcasting system, a video signal of 1/30 second duration and an audio signal of 1/15 second duration are transmitted alternately. The video signal is transmitted in a horizontal scanning period of 1/f.sub. H 1/ f.sub.(.apprxeq. 65.3 .mu.s) in the same manner as a standard television broadcasting signal during the video signal transmission period. The video signal can represent one picture during 1/30 period so that each different picture can be transmitted at each video signal transmission period.
During the audio signal transmission period, a sampled audio signal is transmitted at a repetition period of 1/f.sub. A which is different from the horizontal scanning period of the video signal. The audio signal is pulse code modulated and a plurality of auido signals are transmitted in a time division multiplex scheme. One audio signal is transmitted at each of the above sampling period of 1/f.sub. A. In the transmission of such a still picture broadcasting signal, in order to increase the number of multiplexed audio signals without widening the required frequency band of the transmission path, the audio signals may be transmitted in a multivalue pulse code modulated signal, for instance as a quarternary code.
The synchronizing signals required for the reproduction of such video signal and audio signal must be transmitted at each 1/f.sub. H period during the video signal transmission period and at each 1/f.sub. A period during the audio signal transmission period. Besides the two kinds of synchronizing signals, a number of synchronizing signals having different repetition periods are required, for instance bit synchronizing signals, synchronized with the modulated pulse series for demodulating the pulse code modulated signal is required. In one mode of such a still picture broadcasting system, all of such synchronizing signals may be formed from pulse coded signals. The pulse coded synchronizing signals are formed of binary pulses. The synchronizing signals contain a blanking period, PCM frame pattern signal (PFP signal), and mode control code signal (MCC signal). The PFP signal is formed by pulses of a predetermined pattern having the same bit timing with the modulated pulse series of the audio multiplex signal. This pulse pattern is common to any one of the synchronizing signals having various repetition periods. Accordingly, the PFP signal can be utilized for the detection of the position of insertion of the synchronizing signals. The MCC signal contains synchronizing signals such as a horizontal synchronizing signal, audio PCM frame synchronizing signal and video frame synchronizing signal, etc., and synchronizing signals for representing the transmission period of the video signal or the audio signal.
In order to derive such synchronizing signals and pulse code modulated audio signal, it is necessary to identify the received signal in a usual manner as is the case in the conventional pulse signal transmission system. In this case, in order to identify a quarternary pulse signal, the tolerance of the deviation of the input signal level with a reference level becomes one third with respect to that of the case of the binary pulse signal.
When transmitting a composite signal such as the still picture broadcasting signal by using a carrier signal, the dc level of the detected composite signal becomes usually unstable due to deviation of gain of the tuner or intermediate frequency circuit of the receiver or due to variation of the ambient temperature. Accordingly, identification error tends to occur when an output signal of the intermediate frequency circuit is dc coupled to an identification circuit.
Also if an ac coupling is used in order to avoid the influence of variation of the dc level, the lower components are not transmitted so that ripples may occur. If such ripples are contained, the tolerance in the identification decreases so that an increase in identification errors is unavoidable.
In order to avoid the above mentioned problems, a peak clamp system using a diode and a capacitor may be used before identification of the signal in a conventional PCM transmission system.
Even if such peak clamp system is employed, there will still remain a deviation in the regenerated dc level depending upon the signal content by reason of the ratio r.sub.b /r.sub.f, the ratio of a forward resistance r.sub. f of the diode and a reverse resistance r.sub. b thereof. Accordingly, such practice does not afford a basic solution and the peak clamp system applied before identification of the input signal including the quarternary PCM signal will not give a satisfactory result.